Since the NTSC (National Television System Committee) color television video signal format was adopted in the early 1950's, there has been little change to it. Along with advancement of technology, the resolution or the clearness of the picture delivered by a color television receiver set has become higher and higher. With the introduction of the comb filter, a modern color television receiver can effectively separate the chroma component from the luminance signal without losing the high frequency details of the picture.
However, due to the 4.2 MHz limitation of the NTSC video bandwidth, about 350 lines of resolution in the scanning direction is near the maximum that a standard NTSC color television receiver can deliver. The luminance portion of the NTSC signal is the key factor in determining the resolution in the scanning direction of the reproduced picture. The more picture elements contained in the luminance signal, the higher becomes the resolution in the scanning direction, but the wider becomes the bandwidth required to transmit such a luminance signal.
In a standard home video cassette recorder (VCR), the luminance signal is recorded separately from the chroma signal. But the frequency difference between the luminance recording carrier and the chroma recording carrier is not large enough to provide much more than about 3 MHz bandwidth for the luminance signal. Therefore, the resolution of a video signal reproduced by such a VCR is restricted or limited to about 240 lines of resolution in the scanning direction.
Attempts or proposals have been made to overcome the resolution limits imposed by the standard NTSC video bandwidth so as to meet the demand that a higher resolution, clearer color picture be provided from color television receivers and VCR's, since modern TV receivers and VCR's have now approached their theoretical performance limits. Unfortunately, all of those attempts or proposals to date of which I am aware share the same fatal shortcoming of not being compatible with the existing NTSC standard signal bandwidth or its format, and thus they would entail the obsolescence of a vast existing amount of conventional TV equipment now in the hands of the American public.
In other words, all those attempts or proposals to overcome resolution limits imposed by the NTSC color TV video signal format would overcome the inherent bandwidth limitation problems by changing the rules of the game, thereby making all existing TV sets obsolete. Some of those attempts or proposals will now be discussed briefly.
The super VHS format VCR moves the luminance recording carrier to a higher frequency, thus obtaining wider bandwidth for the luminance signal, while there is no change to the chroma signal. Furthermore, there is required a new video connector (S connector) in which the luminance signal and the chroma signal are separately transmitted so as to bypass the limit of the standard NTSC composite format, thereby enabling a color television monitor to produce a clearer picture from a video cassette. By combining these methods, super VHS is claimed to provide 425 lines of resolution in the scanning direction, which indeed is significantly higher than the 350 lines that the standard NTSC broadcast video signal can deliver.
But, super VHS entails some serious technological trade-offs. Due to the use of a higher luminance recording carrier frequency, a new kind of recording tape has to be used to record in this new super VHS format. A more serious technological penalty is that the tape recorded in the new format can not be played on a standard VCR. It is true that by using a compatible mode, a super VHS VCR can play today's standard tapes, but a new super VHS format tape will not play in today's standard VCR. Another technological penalty is tat the resolution of the picture will suffer if the output from such a new format tape is not taken from the S connector but is taken from the standard NTSC composite video output.
Some high definition television systems have been demonstrated. Such suggested new systems simply employ a wider video bandwidth to accommodate the increased picture element rate and increased number of scanning lines. Such new systems double the resolution both in the horizontal and vertical directions, but the resultant video formats are totally incompatible with the current NTSC standard. The technological penalty is that all of the existing video equipment, existing receivers and existing tape libraries would become obsolete and would need to be replaced at an enormous cost.
There is a frame differential television method which only transmits the difference between adjacent frames of a motion picture. A frame memory in the TV receiver stores the whole image of the last frame. Then, the circuits receive and update only those particular picture elements which are different from corresponding elements in the last frame. Due to the fact that successive frames of a TV motion picture will mostly look alike, the information transmitted per frame for updating each successive frame is reduced as compared with transmitting a whole new frame. But a standard NTSC receiver can not reproduce any picture from those frame differential TV method signals. Therefore, no compatibility with the existing NTSC standard would be provided by the frame differential TV method.
In summary, all of the prior art attempts or proposals now known to me to be seeking to increase the resolution of color television reception suffer from serious compatibility problems. Though these prior art proposals do provide various degrees of improvement in resolution over the current NTSC standard, none of them may coexist and share existing TV or VCR equipment with the current standard.